It is well known in the art to add fillers and/or reinforcing agents to thermally processed polymers (thermoplastic resins). These thermoplastic resins, such as low or high density polyethylene, polypropylene, polycarbonate, nylon, etc. as well as numerous co-polymers such as acrylonitrile-butadiene-styrene etc. are normally processed by heated screw extrusion and may be extruded into shapes or molded via conventional injection or blow molding processes. By incorporating additives into a thermoplastic resin before or during processing in a thermoplastic extruder, a homogeneous mixture of additives and melted thermoplastic resin is formed. This plastic mixture may then be extruded through a die to form continuous shapes, sheets or films, or may be injected into a mold as in an injection molding process to directly form parts of various shapes and configurations, or may be extruded into a parison which is expanded into a mold in a blow molding process. These additives usually consist of fillers such as talc, wood flour, starch, calcium carbonate or flame retardants such as aluminum trihydrate or halogenated compounds, or reinforcing agents such as mica or glass or graphite fibers, or other functional additives, such as colorants, pigments, extenders, blowing agents, surface modifiers, density modifiers, impact modifiers, thermal and electrical conductivity modifiers, or a combination of these ingredients.
It is highly desirable to be able to incorporate additives in thermoplastic resins for many end use applications, in that the finished part can have a lower organic plastic content, which may in turn mitigate recycling problems, lower the propensity of the part to burn, and employ less raw materials from non-renewable sources such as petroleum. In addition, stronger parts with better physical characteristics such as higher heat distortion temperatures can be produced, which allows the use of thinner, lighter parts for many applications such as automotive use, wherein these lighter parts translate to higher fuel mileage for vehicles using them. The incorporation of flame retardant additives imparts flame retardancy which is required for numerous applications, and pigments and colors are usually added for aesthetic reasons.
The amount of additive that can be incorporated into a finished part is limited by the increase in effective melt viscosity experienced when incorporating the additive into the thermoplastic resin, and by the ability of the resin to wet out, and thus contain in a continuous phase, additional amounts of additive. Unwetted additive particles contained in the compound will adversely affect the physical properties of the finished part. As the additive content of the mixture is increased, the melt viscosity of the compound increases and makes it more difficult to produce the desired parts. In injection molding, for instance, increasing the viscosity of the melt mix beyond a certain point causes a problem in that the extremities of the mold cavities may not be properly filled, and areas of finer detail in the mold will not be properly reproduced. In extrusion processes, additive content over a certain maximum will cause a poor surface, and make the hot newly formed extrusion extremely difficult to handle since it will be prone to discontinuities when deflected during handling. Poor processing rheology in the form of high effective viscosity also hinders transport of the additive/thermoplastic resin mix within the extruder and through downstream equipment, such as check valves and hot manifold systems. Also, fire retardant additives will not be properly dispersed, rendering them less effective.
In the production of compounded concentrates, such as color concentrates, large amounts of dyes or pigments are processed with a thermoplastic resin melt to produce high color content particles or pellets. A portion of these color concentrate pellets can then be later employed to color a natural color resin in subsequent thermoplastic processes by combining an amount of these color concentrate pellets with natural color resin pellets and feeding this mixture to an injection molding machine or other heated screw process, wherein the individual pellets in the mixture are melted and combined to form the desired color in the finished part. In these instances, the degree of dispersion of the individual pigment particles within the resin relates to the effective utilization of the pigment in its final intended use, that is, in the coloring of a finished part. Where dispersion is poor, more pigment must be employed to achieve a given color intensity. Extremely fine pigment particles tend to agglomerate, and although the outer surfaces of the agglomerates may be wetted with resin, the individual particles in the agglomerates are not, thus lowering the effective surface area of the pigment and its effectiveness for its intended purpose. In many instances waxes and/or stearates, which are not normally desired in a finished article, must be added in amounts up to 10% or higher to wet out pigments.
Raising the compounding temperature to improve processing rheology can lower the viscosity of the resin and thus help in the wetting out of additives, but this approach has limitations due to the heat sensitivity of the resin and possibly of some of the additives, such as heat sensitive organics or hydrated fire retardant fillers or other ingredients in the compound, and higher temperatures result in longer molding cycle times.
A number of issued patents can be noted which are of interest. U.S. Pat. No. 5,041,259 produces filled and colored thermoplastic resin compositions by employing a process involving multiple addition and mixing steps.
U.S. Pat. No. 4,891,399 relates to the treatment of fillers with silane and titanate coupling agents and silicone fluids for incorporation into a thermoplastic resin.
U.S. Pat. No. 4,810,733 relates to the production of color concentrates wherein large amounts of non-thermoplastic resin components including waxes and/or stearates must be added to the concentrate.
U.S. Pat. No. 4,551,485 treats reinforcing fillers with organic functional silane.
U.S. Pat. No. 4,525,494 relates to the pretreatment of alumina trihydrate with isostearic acid and then compounding the treated alumina trihydrate with a titanate coupling agent and a thermoplastic resin.
U.S. Pat. No. 4,417,018 treats a flame retardant filler with alkoxysilane as a coupling agent for inclusion into a thermoplastic resin.
U.S. Pat. No. 4,374,641 employs solvents in the preparation of color concentrates which must be removed before use.
U.S. Pat. Nos. 4,317,765, 4,071,494 and 3,956,230 compatibilize a filler with a thermoplastic resin by mixing the two in the presence of a peroxide catalyst and maleic anhydride and this mixture is subsequently mixed with additional resin.
U.S. Pat. No. 4,244,860 employs silane additives to prevent the separation of fillers and additives from thermoplastic molding compounds during processing.
The above described examples of prior art along with numerous other patents also describe similar complicated mixing or coupling agent systems. One of the major drawbacks which many of these approaches share is the necessity of pretreating the additive before incorporating it into a thermoplastic resin. This pretreatment is often an elaborate process, usually requiring high shear mixing, solvent or diluent application, and subsequent filler drying, which processing steps add a great deal of cost to the finished product.